Doublecortin expression in the adult rat telencephalon

Doublecortin (DCX) is a protein required for normal neuronal migration in the developing cerebral cortex, where it is widely expressed in both radially and tangentially migrating neuroblasts. Moreover, it has been observed in the adult rostral migratory stream, which contains the neuronal precursors traveling to the olfactory bulb. We have performed DCX immunocytochemistry in the adult rat brain to identify precisely the neuronal populations expressing this protein. Our observations confirm the presence of DCX immunoreactive cells with the characteristic morphology of migrating neuroblasts in the subventricular zone, rostral migratory stream and the main and accessory olfactory bulbs. We have also found putative migratory cells expressing DCX in regions were no adult neuronal migration has been described, as the corpus callosum, the piriform cortex layer III/endopiriform nucleus and the striatum. Surprisingly, many cells with the phenotype of differentiated neurons were DCX immunoreactive; e.g. certain granule neurons in the hilar border of the granular layer of the dentate gyrus, some neuronal types in the piriform cortex layer II, granule and periglomerular neurons in the main and accessory olfactory bulbs, and isolated cells in the striatum. Almost all DCX immunoreactive cells also express the polysialylated form of neural cell adhesion molecule and have a similar distribution to rat collapsin receptor-mediated protein-4, two molecules involved in neuronal structural plasticity. Given these results, we hypothesize that DCX expression in differentiated neurons could be related to its capacity for microtubule reorganization and that this fact could be linked to axonal outgrowth or synaptogenesis.

Developmental changes of synaptojanin expression in the human cerebrum and cerebellum

Synaptojanin is a highly abundant polyphosphoinositide phosphatase in nerve terminals, and has been thought to play roles in clathrin-mediated synaptic vesicle endocytosis and signaling. In order to determine the broader role of synaptojanin in the central nervous system, we examined synaptojanin expression in the cerebrum and cerebellum from the fetal to the adult period by means of immunohistochemical and Western blot analyses. Immunohistochemistry consistently revealed the localization of synaptojanin in Cajal--Retzius cells, cortical plate neurons, subplate neurons, intermediate neurons, germinal matrix cells and the ventricular neuroepithelium of the fetal cerebrum. In the fetal cerebellum, synaptojanin immunoreactivity was localized in the external granular cell layer, Purkinje cell layer neuropil, cytoplasm of Purkinje cells and internal granular cells. The immunoreactivity in these structures was decreased around birth. After birth, the synaptojanin immunoreactivity of cortical neurons in the cerebrum, Purkinje cell layer neuropil, and internal granular cells and Purkinje cells in the cerebellum increased and reached a plateau after 11 years of age. These results were consistent with the intensity observed on Western blot analysis. These developmental changes of synaptojanin suggest a broader role in not only synaptic vesicle recycling, but also the regulation of neuronal migration and synaptogenesis in the fetal cerebrum and cerebellum.

Prepubertal increases in the levels of two salmon gonadotropin-releasing hormone mRNAs in the ventral telencephalon and preoptic area of masu salmon

Ontogenic changes in the expression levels of two salmon gonadotropin-releasing hormone genes (sGnRH-I and -II) were examined in the forebrain region including the ventral telencephalon and preoptic area of masu salmon by competitive reverse transcription-polymerase chain reaction (RT-PCR). Two genes showed similar expression patterns throughout the lifetime in both sexes, although the levels of sGnRH-II mRNA were about 20 times higher than those of sGnRH-I mRNA. In males, the levels of sGnRH mRNAs increased at the beginning of the second year and reached their maximum in the autumn. The levels decreased gradually until the autumn of the third year when fish sexually matured. In females, the levels reached their maximum in the first autumn and fluctuated considerably along with the seasons in the third year. These results suggest that, in the salmon brain, sGnRH genes are activated long before the sexual maturation under sexually different control mechanisms.

A mutation in the Gsk3-binding domain of zebrafish Masterblind/Axin1 leads to a fate transformation of telencephalon and eyes to diencephalon

Zebrafish embryos homozygous for the masterblind (mbl) mutation exhibit a striking phenotype in which the eyes and telencephalon are reduced or absent and diencephalic fates expand to the front of the brain. Here we show that mbl(-/-) embryos carry an amino-acid change at a conserved site in the Wnt pathway scaffolding protein, Axin1. The amino-acid substitution present in the mbl allele abolishes the binding of Axin to Gsk3 and affects Tcf-dependent transcription. Therefore, Gsk3 activity may be decreased in mbl(-/-) embryos and in support of this possibility, overexpression of either wild-type Axin1 or Gsk3beta can restore eye and telencephalic fates to mbl(-/-) embryos. Our data reveal a crucial role for Axin1-dependent inhibition of the Wnt pathway in the early regional subdivision of the anterior neural plate into telencephalic, diencephalic, and eye-forming territories.

Early-generated preplate neurons in the developing telencephalon: inward migration into the developing striatum

Specialized subsets of early-generated neurons provide the cellular cues that are necessary for the establishment of characteristic cell and fiber interactions in each brain region. During the development of the mammalian cerebral cortex, the early-generated cells line up in the most superficial part of the telencephalic pallium forming the preplate. It has been generally thought that the preplate derivatives are exclusively located in the cortical region and govern the early histogenetic phase of cortical development. However, we here disclose an unexpected evidence that a subset of early-generated neurons of the piriform preplate migrate inward into and disperse within the subcortical structure striatum during the embryonic stage. Their migratory route is unique and its direction is opposite to the ordinary migration of neuronal precursors directed outward from the periventricular germinal zone. After immigrating into the developing striatum, these early-generated cells are closely associated with the intrastriatal fascicules of axons. The majority of these cells are eliminated by apoptotic cell death during the early postnatal stage. Based on these findings, we propose a new concept: the preplate neurons may not only direct cortical histogenesis but also change their location to play a role in the development of subcortical structures.

Microstructural brain development after perinatal cerebral white matter injury assessed by diffusion tensor magnetic resonance imaging

OBJECTIVE

Brain injury in premature infants is characterized predominantly by perinatally acquired lesions in the cerebral white matter (WM). The impact of such injury on the subsequent development of cerebral WM is not clear. This study uses diffusion tensor magnetic resonance imaging (MRI) to evaluate the effects of cerebral WM injury on subsequent microstructural brain development in different WM areas of the brain.

METHODS

Twenty premature infants (gestational age: 29.1 +/- 1.9 weeks) were studied by conventional MRI within the first 3 weeks of life and again at term, with the addition at the latter time of diffusion tensor MRI. Ten of the preterm infants had cerebral WM injury identified by the early MRI and were matched with 10 premature infants of similar gestational age and neonatal course but with normal neonatal MRI scans. Diffusion tensor MRI at term was acquired in coronal and axial planes and used to determine the apparent diffusion coefficient, a measure of overall restriction to water diffusion, and the relative anisotropy (RA), a measure of preferred directionality of diffusion, in central WM, anterior frontal WM, occipital WM, temporal WM, and the posterior limb of the internal capsule. Diffusion vector maps were generated from the diffusion tensor analysis to define the microstructural architecture of the cerebral WM regions.

RESULTS

At term, the diffusion tensor MRI revealed no difference in apparent diffusion coefficient among preterm infants with or without perinatal WM lesions. By contrast, RA, the measure of preferred directionality of diffusion and thereby dependent on development of axonal fibers and oligodendroglia, was 25% lower in central WM, the principal site of the original WM injury. However, RA was unaffected in relatively uninjured WM areas, such as temporal, anterior frontal, and occipital regions. Notably, RA values in the internal capsule, which contains fibers that descend from the injured cerebral WM, were 20% lower in the infants with WM injury versus those without. Diffusion vector maps showed striking alterations in the size, orientation, and organization of fiber tracts in central WM and in those descending to the internal capsule.

CONCLUSIONS

Perinatal cerebral WM injury seems to have major deleterious effects on subsequent development of fiber tracts both in the cerebral WM and more distally. The ultimate impact of brain injury in the newborn should be considered as a function not only of tissue destruction, but also of impaired subsequent brain development.

Intrinsic and extrinsic control of cortical development

Recent advances in the study of cerebral cortical early development are described in this chapter. The role of the anterior neural ridge in regulating telencephalon induction in the neural plate is discussed, followed by a review of the evidence for the roles of ventral, rostral and dorsal patterning centres in regulating regionalization of the telencephalon. The patterning centres produce secreted molecules (SHH, FGF, BMP, WNT) that regulate the expression of transcription factors which control regional identity, cell type specification, proliferation and differentiation. These intrinsic patterning mechanisms appear to be sufficient to generate much of the regional organization of the cerebral cortex present in newborn mice. While intrinsic mechanisms have a major role in cortical regionalization and in the production of cortical projection neurons, many cortical interneurons are derived from the basal ganglia and then migrate into the cerebral cortex. Furthermore, thalamic afferents appear to have an important role in maturation of the postnatal rodent cortex. Thus, both intrinsic and extrinsic mechanisms control development of the cerebral cortex.

Lesions of a telencephalic nucleus in male zebra finches: Influences on vocal behavior in juveniles and adults

Male zebra finches learn to sing during a restricted phase of juvenile development. Song learning is characterized by the progressive modification of unstable song vocalizations by juvenile birds during development, a process that leads to the production of stereotyped vocal patterns as birds reach adulthood. The medial magnocellular nucleus of the anterior neostriatum (mMAN) is a small cortical region that has been implicated in song behavior based on its neuronal projection to the High Vocal Center (HVC), a nucleus that is critical for adult vocal production and presumably also plays a role in song learning. To assess the function of mMAN in song, ibotenic acid lesions of this brain region were made in juvenile male zebra finches during the period of vocal learning (40-50 days of age) and in adult males that were producing stable song (>90 days of age). Birds lesioned as juveniles produced highly abnormal, poor quality song as adults. Although the overall song quality of birds lesioned as adults was not highly disrupted or abnormal, the postoperative song behavior of these birds was discernibly different due to slight increases in variability of vocal production, particularly at the onset of singing. These results demonstrate that mMAN plays some important role in vocal production during the sensitive period for song learning, and is also important for consistent initiation and stereotyped production of adult song behavior.

The nodal pathway acts upstream of hedgehog signaling to specify ventral telencephalic identity

The Nodal and Hedgehog signaling pathways influence dorsoventral patterning at all axial levels of the CNS, but it remains largely unclear how these pathways interact to mediate patterning. Here we show that, in zebrafish, Nodal signaling is required for induction of the homeobox genes nk2.1a in the ventral diencephalon and nk2.1b in the ventral telencephalon. Hedgehog signaling is also required for telencephalic nk2.1b expression but may not be essential to establish diencephalic nk2.1a expression. Furthermore, Shh does not restore ventral diencephalic development in embryos lacking Nodal activity. In contrast, Shh does restore telencephalic nk2.1b expression in the absence of Nodal activity, suggesting that Hedgehog signaling acts downstream of Nodal activity to pattern the ventral telencephalon. Thus, the Nodal pathway regulates ventral forebrain patterning through both Hedgehog signaling-dependent and -independent mechanisms.

Transient expression of SorCS in developing telencephalic and mesencephalic structures of the mouse

Here we describe the expression of a third member of the VPS10 domain containing receptor family, SorCS, during mouse embryonal and early postnatal nervous system development. SorCS is expressed in a unique transient and dynamic pattern in regions where cells proliferate, as well as in areas where already differentiated cells reside, including the cerebral cortex, the ventral tegmental area, and the globus pallidus. Transcripts were absent from fiber tracts hinting at a neuronal expression. The only exception was hybridization signals on the developing optic nerve correlating with the appearance of astrocytes migrating into the retina.

Insulin-like growth factor-I promotes neurogenesis and synaptogenesis in the hippocampal dentate gyrus during postnatal development

The in vivo actions of insulin-like growth factor-I (IGF-I) on the growth and development of the hippocampal dentate gyrus were investigated in transgenic mice that overexpress IGF-I postnatally in the brain and in normal nontransgenic littermate controls. Stereological analyses of the dentate gyrus were performed by light and electron microscopy on days 7, 14, 21, 28, 35, and 130 to determine postnatal changes in the numerical density and total number of neurons and synapses. The volumes of both the granule cell layer and the molecular layer of the dentate gyrus were significantly increased by 27-69% in transgenic mice after day 7, with the greatest relative increases occurring by day 35. Although the numerical density of neurons in the granule cell layer did not differ significantly between transgenic and control mice at any age studied, the total number of neurons was significantly greater in transgenic mice by 29-61% beginning on day 14. The total number of synapses in the molecular layer was significantly increased by 42-105% in transgenic mice from day 14 to day 130. A transient increase in the synapse-to-neuron ratio was found in transgenic mice at postnatal days 28 and 35 but not at day 130. This finding indicates a disproportionate increase in synaptogenesis, exceeding that expected for the observed increase in neuron number. Our results demonstrate that IGF-I overexpression produces persistent increases in the total number of neurons and synapses in the dentate gyrus, indicating that IGF-I promotes both neurogenesis and synaptogenesis in the developing hippocampus in vivo.

Alpha 1E subunit of the R-type calcium channel is associated with myelinogenesis

During myelinogenesis, we found an exceedingly strong, transient expression of the alpha(1E) gene for the R-type voltage-gated calcium channel in CNS white matter. This immunoreactivity appeared in glial cells along specific pathways of the brainstem, cerebellum, and telencephalon. The reactivity followed a wave that progressed from the brainstem at P5, to the cerebellar peduncles by P8, the arbor vitae by P14, and the granular layer by P17. The reactivity-peaked about 3-4 days later and decreased gradually to become negligible in all areas before adulthood. Ultrastructural analysis confirmed that alpha(1E) immunoreactivity was located in oligodendroglial somata, their projections, paranodal wraps and loose myelin sheaths. There was a distinct association of the channel protein reactivity on oligodendroglial membranes in contact with the axon. We propose that glial projections, contacting axons, sense axonal firing through small K(+) currents and open the high voltage R-type calcium channels to signal myelination.

Widespread expression of rat collapsin response-mediated protein 4 in the telencephalon and other areas of the adult rat central nervous system

The rat collapsin response-mediated protein 4 (rCRMP-4) is a member of a family of proteins that are involved in axonal growth. It is found transiently in postmitotic neurons, such as those that are generated in the adult hippocampus. The authors used immunocytochemistry to investigate whether areas of the rat central nervous system (CNS) that retain postnatal neurogenesis express this protein. They found pronounced rCRMP-4 immunoreactivity in recently generated cells in the dentate granular layer, the subventricular zone, the olfactory bulbs, and the rostral migratory stream, four areas in which the production or migration of neurons occurs in adulthood. However, rCRMP-4 immunoreactivity also is expressed in many other regions of the rat brain in which there is no record of adult neurogenesis or neuronal migration, e.g., in the olfactory glomeruli and in neurons of the cerebral cortex. In the hypothalamus, intensely rCRMP-4-labeled neurons populated the supraoptic, paraventricular, and periventricular nuclei as well as the median eminence and the arcuate nucleus. Immunoreactivity for rCRMP-4 also was present in certain neurons of the interpeduncular nucleus, median raphe, superior colliculus, and scattered granule cerebellar neurons. Many of these regions are known to display axonal outgrowth and/or synaptic rearrangement in adulthood and to coexpress the polysialylated form of the neural cell adhesion molecule. Thus, the results of this study suggest that rCRMP-4 expression in the CNS is associated with cells that are migrating or are undergoing axonal growth. Nevertheless, small, rCRMP-4-immunoreactive cells were seen throughout the brain. These cells did not express neuronal, astroglial, or microglial markers, although some of them also were immunoreactive for rip antibody, suggesting an oligodendroglial lineage.

Reorganization of a telencephalic motor region during sexual differentiation and vocal learning in zebra finches

Male zebra finches learn to produce a vocal pattern during a sensitive period of development, whereas females do not. The motor output of telencephalic song processing is RA (the robust nucleus of the archistriatum), a region containing a population of projection neurons that descend to the hindbrain (nXIIts, the tracheo-syringeal portion of the hypoglossal nerve nucleus). In turn, nXIIts neurons innervate the vocal organ (syrinx). Previous work shows that the number of RA neurons is monomorphic in fledgling males and females. RA neuron number in males does not change thereafter, but females show a substantial developmental loss of RA neurons. Because the developmental sex difference in RA neuron number implies a change in the number of projection neurons within RA, we have made injections of retrograde tracer into the hindbrain to measure the percentage of RA neurons that project to nXIIts as a function of age in females and vocal development in males. In juveniles of both sexes, we found that close to two-thirds of RA neurons project to nXIIts. However, the percentage of RA neurons projecting to nXIIts declined by 44% during female development, and by 14% during a specific stage of male vocal development (the transition from subsong to plastic song). These data indicate that in addition to regulation of RA neuron number, sexual differentiation and vocal learning correlate with a significant decrease in the amount of descending input to the vocal organ. The loss of projection neurons during vocal learning is surprising in light of the stability of RA neuron number in males, and our findings suggest that the behavioral transition from subsong to plastic song may involve a restricted period of RA neuron loss and replacement and/or axon rearrangement.

Expression of Meis and Pbx genes and their protein products in the developing telencephalon: implications for regional differentiation

The Meis and Pbx genes encode for homeodomain proteins of the TALE class and have been shown to act as co-factors for other homeodomain transcription factors (Mann and Affolter, 1998. Curr. Opin. Genet. Dev. 8, 423-429). We have studied the expression of these genes in the mouse telencephalon and found that Meis1 and Meis2 display region-specific patterns of expression from embryonic day (E)10.5 until birth, defining distinct subterritories in the developing telencephalon. The expression of the Meis genes and their proteins is highest in the subventricular zone (SVZ) and mantle regions of the ventral telencephalon. Compared to the Meis genes, Pbx genes show a broader expression within the telencephalon. However, as is the case in Drosophila (Rieckhof et al., 1997. Cell 91, 171-183; Kurrant et al., 1998. Development 125, 1037-1048; Pai et al., 1998. Genes Dev. 12, 435-446), nuclear localized PBX proteins were found to correlate highly with Meis expression. In addition, DLX proteins co-localize with nuclear PBX in distinct regions of the ventral telencephalon.

Ontogenic profile of the expression of the mu opioid receptor gene in the rat telencephalon and diencephalon: an in situ hybridization study

The developmental profile of mu (mu) opioid receptor gene expression has been characterized in the embryonic, postnatal and adult rat brain by in situ hybridization histochemistry. By ED12, mu opioid receptor mRNA was detectable in the deep neuroepithelium of the cortical plate. In the developing rat central nervous system (ED13-PD40), transcripts were seen over numerous telencephalic and diencephalic structures, such as the olfactory bulb, caudate-putamen, nucleus accumbens, amygdaloid complex, hippocampal formation, hypothalamus and thalamus. In the vast majority of brain regions examined, the developmental profile of the mu opioid receptor gene expression is similar to that of its translated protein as established using receptor autoradiography. Once a hybridization signal is detected in the prenatal period, it gradually increased to reach maximal levels during the second and third postnatal weeks. By the end of the third postnatal week, mu opioid receptor mRNA levels decreased to reach amounts seen in adulthood. Our study demonstrates that mu opioid receptor gene expression is seen very early on in the embryonic rat brain with transient increases observed during the critical period of neurogenesis, neuronal migration and synaptogenesis, suggesting a role of this opioid receptor subtype in brain developmental processes.

Expression of PTTG and prc1 genes during telencephalic neurogenesis

We present the first time/space analysis using in situ hybridization for PTTG and prc1 genes during development of the mouse telencephalon. During the stages E11.5-E13.5 PTTG and prc1 are expressed in most tissues of the embryo. Within the telencephalon, PTTG and prc1 are found exclusively inside of the ventricular zone (VZ). The intensity of the expression of both genes in the ventricular zone reaches its peak by E15.5. Expression starts to decrease by E18.5, it is still visible at least up to P2 and not detectable in the adult brains. Expression of the prc1 gene, but not that of the PTTG, is also found in the mitoticaly active cells outside of the VZ within the telencephalon. Most of the cells expressing the PTTG gene were found in the lower part of the ventricular zone suggesting that the level of PTTG mRNA is regulated during different phases of the mitotic cycle.

Increased expression of endogenous biotin, but not BDNF, in telencephalic song regions during zebra finch vocal learning

Brain-derived neurotrophic factor (BDNF) is thought to regulate multiple aspects of brain development and neural plasticity in vertebrates. We have examined BDNF expression in two telencephalic nuclei (RA and HVC) in the zebra finch brain that control song learning by juvenile males and the production of already-learned song by adults. Using two different antibody-labeling techniques (avidin-biotin complex and horseradish peroxidase), we were unable to detect BDNF-like immunoreactivity in RA of juvenile or adult birds. BDNF-like immunoreactive labeling of somata was detected in HVC, but the density of labeled cells was not different between juvenile and adult birds. Immunocytochemical findings were confirmed by RT-PCR for BDNF mRNA. Thus, in contrast to a previous report (Akutagawa and Konishi, Proc. Natl. Acad. Sci. USA 95 (1998) 11429-11434), we did not observe elevated levels of BDNF immunoreactivity in RA and HVC of juvenile birds that were learning to sing. However, RA and HVC of juvenile birds were found to express elevated levels of endogenous biotin (as detected by avidin peroxidase), suggesting a possible role for biotin-regulated mechanisms in songbird vocal learning.

Expression of L1 and TAG-1 in the corticospinal, callosal, and hippocampal commissural neurons in the developing rat telencephalon as revealed by retrograde and in situ hybridization double labeling

In the telencephalon, the corticospinal (CS), callosal, and hippocampal commissural neurons are the major types of neurons that have axons crossing the midline of the brain. To understand the mechanisms involved in crossing the midline structure and to examine whether the expression patterns of L1 and TAG-1 in the commissural neurons are similar to those in the spinal cord, we investigated L1 and TAG-1 expression in these neurons in rats by using a double-labeling technique involving retrograde labeling and in situ hybridization. Expression of L1 messenger RNA was detected in the retrogradely labeled CS projection neurons by 1,1;-dioctadecyl-3,3, 3;,3;-tetramethylindocarbocyanine perchlorate (DiI) injection into the pons at embryonic day (E) 19, but expression of TAG-1 messenger RNA was not detected in these neurons. Also, after their axons crossed the pyramidal decussation, continued expression of L1 but no expression of TAG-1 in the CS projection neurons was shown by an additional double-labeling experiment involving DiI injection into the spinal cord at postnatal day (P) 1. An immunohistochemical study showed that L1 was continuously present in each level of the CS tract at E21 and P3, but TAG-1 immunoreactivity was not found in any level at any stage. Finally, we examined the expression of L1 and TAG-1 messenger RNAs in the callosal and hippocampal commissure neurons after their axons had crossed the midline by using the double-labeling technique. In both cases, hybridization signals of the L1 and TAG-1 messenger RNAs were observed in the retrogradely labeled neurons at P3. These results suggest that the roles of L1 and TAG-1 in the formation of the commissures in the forebrain are different from their roles in the spinal cord.

Expression and distribution of various antigens of developing microglial cells in the rat telencephalon

The distribution of microglia during the early stages of postnatal development in the rat was studied on rat brain from day of birth to postnatal day 90 (P90), using immunohistochemical methods with a panel of monoclonal antibodies that recognized the complement type 3 receptor (OX-42), macrophage antigen of unknown function (ED1), and the major histocompatibility complex (MHC) class I (OX-18) or class II (OX-6) antigens. Starting from the day of birth, ameboid microglia can be differentiated with positive immunoreactivity to OX-42, OX-18, and ED1. Labeled cells were localized mainly in the developing white matter. After P21, only positive reaction to OX-42 was present, and those cells had the typical morphology of the resting microglial cells that were located either in the white or grey matter. The changes in the appearance of different antigens are correlated with the morphological differentiation and transformation of ameboid microglial cells that are to become ramified microglia, present in the adult animals.

A field study of seasonal neuronal incorporation into the song control system of a songbird that lacks adult song learning

Adult songbirds can incorporate new neurons into HVc, a telencephalic song control nucleus. Neuronal incorporation into HVc is greater in the fall than in the spring in adult canaries (open-ended song learners) and is temporally related to seasonal song modification. We used the western song sparrow, a species that does not modify its adult song, to test the hypothesis that neuronal incorporation into adult HVc is not seasonally variable in age-limited song learners. Wild song sparrows were captured during the fall and the spring, implanted with osmotic pumps containing [3H]thymidine, released onto their territories, and recaptured after 30 days. The density, proportion, and number of new HVc neurons were all significantly greater in the fall than in the spring. There was also a seasonal change in the incorporation of new neurons into the adjacent neostriatum that was less pronounced than the change in HVc. This is the first study of neuronal recruitment into the song control system of freely ranging wild songbirds. These results indicate that seasonal changes in HVc neuronal incorporation are not restricted to open-ended song learners. The functional significance of neuronal recruitment into HVc therefore remains elusive.

Fate of new neurons in adult canary high vocal center during the first 30 days after their formation

Projection neurons are added to the high vocal center (HVC) of adult songbirds. Here we report on events associated with their initial arrival in HVC. Neurons formed in adult canaries were labeled with [(3)H]-thymidine and examined 8, 15, 22, and 31 days later. By 8 days, some [(3)H]-labeled cells with the nuclear profile of postmigratory neurons were already present in HVC but could not be retrogradely labeled by Fluoro-Gold injections in the robust nucleus of the archistriatum (RA); 7 days later, a few such cells could be backfilled from RA. Thus, new neurons may arrive in HVC as much as 1 week prior to establishing connections with RA. By 31 days, 43% of the [(3)H]-labeled neurons could be backfilled from RA. In no case were new neurons backfilled by tracer injections into Area X, suggesting that newly formed HVC cells do not establish a transient connection with this region. At all survival times, the somata of new neurons were often clustered tightly together with other HVC neurons that differed in age and projection. Between days 15 and 25 after their birth, half of the new HVC neurons disappeared. We conclude: (1) that neurons arrive in HVC earlier than previously thought, (2) that soon after their arrival they become part of cell clusters in HVC, and (3) that in addition to the previously described death of new neurons that occurs over a period of months, there is an early wave of death that occurs soon after new neurons adopt a postmigratory phenotype.

Early neocortical regionalization in the absence of thalamic innervation

There is a long-standing controversy regarding the mechanisms that generate the functional subdivisions of the cerebral neocortex. One model proposes that thalamic axonal input specifies these subdivisions; the competing model postulates that patterning mechanisms intrinsic to the dorsal telencephalon generate neocortical regions. Gbx-2 mutant mice, whose thalamic differentiation is disrupted, were investigated. Despite the lack of cortical innervation by thalamic axons, neocortical region-specific gene expression (Cadherin-6, EphA-7, Id-2, and RZR-beta) developed normally. This provides evidence that patterning mechanisms intrinsic to the neocortex specify the basic organization of its functional subdivisions.

Interactions between nerve growth factor binding and estradiol in early development of the zebra finch telencephalon

The zebra finch telencephalon exhibits rapid and substantial development in the first few weeks after hatching. In parallel, the rate of estradiol synthesis is very high in the zebra finch forebrain, and estradiol can have potent neurotrophic effects in specific telencephalic regions, including those that control the learning and production of song. In an attempt to elucidate mechanisms regulating telencephalic development, potentially including a role for the large capacity for estrogen production, (125)I-nerve growth factor (NGF) binding was measured in homogenates of telencephalon from zebra finches age 3, 15, 30, 60, and 120 days. The highest density of low- and high-affinity (125)I-NGF binding sites was observed in 3-day-old finches. Using an aromatase inhibitor, Fadrozole, to reduce estradiol levels in 1 to 4-day-old zebra finches significantly decreased both high- and low-affinity (125)I-NGF binding sites. Conversely, treating adult or 8 to 14-day-old hatchlings with estradiol increased high-affinity (125)I-NGF binding sites. These results are consistent with the hypothesis that estradiol influences the level of NGF receptors, and suggest one mechanism through which the steroid could affect brain development. The data also indicate that estradiol and NGF activity may be important for very early development of the telencephalon.